The so-called sputtering apparatus for forming a film by sputtering a target as a thin film forming element in plasma has been used widely in various fields of forming thin films of various materials. Especially (1) a conventional diode (rf or dc) sputtering apparatus (refer to F. M. D'HEURLE: Metall. Trans., Vol 1, March, 1970, pp 725-732) in which a target 1 and a substrate 2 are disposed in opposing relationship in a vacuum chamber 4 as shown in FIG. 1; (2) a triode sputtering apparatus (refer to W. W. Y. Lee and D. Oblas: J. Appl. Phys., Vol. 46, No. 4, 1975 pp 1728-1732) in which, as shown in FIG. 2, a third electrode 3 for emitting electrons is further disposed and (3) the magnetron sputtering process (refer to R. K. Waits: J. Vac. Sci. Technol., Vol. 15, No. 2, 1978, pp 179-187) in which, as shown in FIG. 3, a suitable magnetic field is applied to a target 1 by a magnet 5 to generate high density and low-temperature plasma, thereby forming thin films at a high rate, are known in the technical field of thin film formation. Each of the above-mentioned equipment and process comprises generally a target 1 which is a thin film forming element, a substrate 2 upon which a thin film is formed, a vacuum chamber 4 in which the target 1 and the substrate 2 are enclosed and gas inlet and outlet systems, whereby plasma is generated in the vacuum chamber 4.
In case of forming a thin film at a high rate in each of the above-mentioned apparatus and process, it is essential that the density of plasma be maintained at a high level. In the case of the diode sputtering apparatus, the higher the density of plasma, the more drastically a voltage applied to the target is increased and concurrently the temperature of the substrate rapidly rises because of the bombardment of high energy particles an high-energy electrons in the plasma thereon. As a result, damage to the grown thin films is increased, so that the diode sputtering apparatus can be only used with special heat-resistive substrates, thin film materials and film compositions. In the case of the triode sputtering apparatus, even though the density of the plasma increases by the feeding of electrons from the third electrode, as in the case of the diode sputtering apparatus, the rapid temperature rise of the substrate results in damage to the thin film. As a consequence, the triode sputtering apparatus has the defect that only special film-forming materials may be used and the kinds of substrates to be used are also limited in number.
On the other hand, with the high-rate magnetron sputtering apparatus, the gamma (.gamma.-) electrons which are emitted from the target and which are needed to ionize a gas in plasma are confined over the surface of the target by the magnetic and electric field, so that it becomes possible to generate high-rate plasma at a low gas pressure. For instance, the high-speed sputtering process can be carried out at a low pressure as low as 10.sup.-3 Torr in practice, so that the magnetron sputtering process has been used widely to form various thin films at high rates. However, in this process, the film being formed is subjected to the bombardment of ions (mainly Ar.sup.+) from the plasma; high-energy neutral particles (mainly Ar reflected at the surface of the target); and negative ions. Accordingly in almost all the cases, the compositions of the films thus formed are deviated and not only the film but also the substrate are damaged. Furthermore, it is well known in the art that in the case of the formation of a ZnO film, the quality of the portion of the ZnO film immediately above the eroded or bombarded portion of the target is considerably different from the quality of the remaining portion. Thus, the magnetron sputtering process presents the problem of bombardment of highenergy particles over the surface of the substrate. In addition, since the eroded or bombarded portion of the target is locally distributed it has the defects that the efficiency of utilization is remarkably low and the productivity is not high in the case of industrialscale formation of thin films.
Furthermore, in the case of the formation of thin films by the conventional sputtering apparatus, the gas and the particles in plasma are not sufficiently ionized, so that neutral particles which are sputtered and deposited on the surface of the substrate are almost neutral when they impinge upon the surface of the substrate. From the standpoint of reaction, a sufficiently high degree of activation cannot be attained, and therefore in order to obtain an oxide film or films of thermally nonequilibrium materials, the temperature of the substrate must be raised to a temperature as high as 500.degree.-800.degree. C. In addition, almost all the electric power supplied to the plasma is consumed as thermal energy and the ratio of the electric power used to form plasma (ionization) to the total power applied to the apparatus is low, so that there is the defect that the power efficiency is low.
Furthermore, with the above-mentioned sputtering apparatus, stable discharge cannot be maintained at a gas pressure lower than 10.sup.-3 Torr and accordingly there arises a problem that impurities are trapped in a formed thin film.
Meanwhile, Matsuo et al. proposed a method for forming thin films (U.S. Pat. No. 4,401,054) by which plasma containing a material to be deposited is generated under electron cyclotron resonance conditions and the plasma thus generated is directed toward a specimen, thereby forming a thin film. However, by this method it is impossible to form a thin metal film or metal-compound film. Matsuo and Ono (U.S. Pat. No. 4,492,620) and Ono et al. (Jpn. J. Appl. Phys., Vol. 23, No. 8, 1884, L534-L536) disclosed, a microwave plasma deposition apparatus in which a target is sputtered by microwave plasma generated by the electron cyclotron resonance (ECR) and the sputtered particles are deposited on the surface of a substrate, thereby forming a thin film.
The microwave plasma has excellent features in that the discharge can be produced at a pressure of from 10.sup.-5 to 10.sup.-4 Torr and the plasma is highly active. This apparatus is superior in that the highly activated plasma is generated at a low gas pressure by utilizing the above-mentioned features of the microwave plasma.
However, the target is disposed outside of the plasma generation chamber, so that the sputtering rate is not high enough and the ratio of ionization of the particles sputtered from the target is low and furthermore the energy control is not satisfactory.
Japanese Patent Application Laid-Open Nos. 61-87869 and 61-194174 disclose apparatus in which microwave plasma produced in ECR condition is confined by a mirror field so that the sputtering process can be carried out by the plasma whose density is considerably increased. In both cases, the sputtering process can be carried out at a high vacuum of the order of 10.sup.-4 -10.sup.-5 Torr. However, in the former apparatus, both the target and the substrate are disposed within high density plasma. Consequently, there arises the that high-energy neutral particles and charged particles directly bombard the surface of the substrate, so that the quality of the thin film being formed is degraded. Furthermore, there exists an adverse effect that the temperature of the substrate is raised by the high density plasma, so that a cooling mechanism for cooling the surface of the substrate becomes complex in construction. In the latter apparatus, plasma generated in a mirror field is extracted toward the surface of a target by another magnetic field generating means so as to sputter the target. In this case, the target is not disposed in the plasma generation chamber in which high density plasma is generated, so that the ratio of the ionization of the particles sputtered from the target is low. Therefore, the latter apparatus is not adapted to the formation of thin films by highly reactive ions.
In the case of the formation of thin films by the sputtering process, it is preferred that the following conditions are satisfied:
(1) The film and the substrate are not damaged, drastic rises in their temperatures must be prevented, and the films are formed at a high-rate (In other words, high density plasma must be generated); PA1 (2) The energy of the particles can be controlled over a wide range; PA1 (3) The dispersion of the energies of the particles must be minimized as practically as possible; PA1 (4) The ionization ratio of plasma is high and the plasma is active; and PA1 (5) Plasma can be generated even at a low gas pressure. PA1 a vacuum chamber consisting of a vacuum waveguide having a microwave introduction window at one end thereof, the microwave introduction window being connected to a microwave waveguide, a plasma generation chamber having a diameter and a length sufficient to define a microwave cavity resonator for causing resonance of the introduced microwave energy, and a specimen chamber all connected with each other in the order named, and further having a gas introduction inlet; PA1 at least one pair of magnetic field generating means which are disposed around the outer side of both the end portions of the plasma generation chamber so that a mirror field is produced in such a way that the center of the magnetic field exists within in the plasma generation chamber; PA1 a pair of targets disposed within and at both ends of the plasma generation chamber perpendicular to the directions of the magnetic fluxes produced by the at least one pair of magnetic field generating means and applied with a negative potential; and PA1 the specimen chamber being connected to the plasma generation chamber in the direction perpendicular to the magnetic fluxes.
In order to satisfy the above-described conditions, the same inventors disclosed in Japanese Patent Application No. 61-66866 a process in which a target is disposed substantially in parallel with the magnetic flux in front of high density plasma generated in a mirror field so that the sputtering process can be carried out at a high degree of efficiency and the ions generated are extracted in the direction of the magnetic field. In this process, the surface of the target is disposed substantially in parallel with the direction of the magnetic flux, so that the ions must be forced to traverse across the magnetic field and impinge on the target when they are extracted toward the target to sputter the same. Therefore, from the standpoint of a plasma generation efficiency, the process must be improved to exhibit a high efficiency. Furthermore, since the surface of the substrate upon which a thin film is formed is disposed perpendicular to the magnetic flux, there still remains a danger that the substrate is subjected to the bombardment of high-energy neutral particles.